Sometimes the best attack is having the best defense of anyone. In this case, Yu-Gi-Oh! has a lot of defensive cards that can also hold up with the greatest attacking cards. Even the cards that match their defense with their mighty attack can either be great weapons or great shields.
Its use depends on you and how you use these strong cards. Defense can be the perfect opportunity to counter, and if you have the strongest, you buy yourself more time to do so as your opponent struggles.
10 Slifer The Sky Dragon (x ATK, x DEF)
Slifer's attack and defense are dependent on the number of cards in your hand, and while the price is high to summon this card, it can have the strongest defense of all cards if given the chance.
It also has the chance to be really weak, which is why it's last on this list. But if you can literally play your cards right you have a fortress that no one can conquer through Slifer. The best part is if you can manage the top level of attack from Slifer you have a strong card to attack with.
9 Exxod, Master of the Guard (0 ATK, 4000 DEF)
While with Slifer you couldn't guarantee its defense, you can with Exxod. While it's obviously inspired from Exodia, Exxod won't win you the match automatically. It's best use is a defensive wall to hold back your opponent while you try to activate its special effect. With its ability, it wouldn't be a bad card to build a deck around. Once activated, if you slip summon an earth type monster card, you get to inflict 1000 damage on your opponent. It can be really entertaining to watch your opponent squirm as you flip a card each turn, basing your deck with enough earth monsters can leave your opponent down in just a couple of turns if played right.
8 A to Z Dragon Buster Cannon (4000 ATK, 4000 DEF)
This card has a guaranteed 4000/4000 stats but has very specific rules for summoning. You must discard from play the relevant which are implied from the name, and summon this card from an extra deck you have, in other words, not the deck you're playing with. To play this is a bit convoluted - but then you get its attack strength and defense.
7 The Wicked Dreadroot (4000 ATK, 4000 DEF)
This, like in its name, has a wicked effect. Its summoning is the equivalent to an Egyptian God Card where you sacrifice 3 monsters, after that this card comes into play and becomes the dominant card on the field. Once it is played it halves the attack and defense of all cards on the field except itself, basically doubling its strength and defense for that turn. This card has one of the best abilities of this list, however, it only lasts one turn and this card is far from invincible.
6 Super Vehicroid - Mobile Base (0 ATK, 5000 DEF)
This is the first of the 5000 stat cards. Along with its overly long and over-the-top name, it also has over-the-top defense. Just beware if it gets put into attack mode because of its 0 attack points, it would easily get destroyed. It does have a special effect that does make up for not having attack. For whatever strength of your opponent's monster, you get to special summon a monster with equivalent attack, meaning you can keep your defensive line going.
5 Malefic Truth Dragon (5000 ATK, 5000 DEF)
This is a good 5000/5000 monster if you can summon it. It will take another card from its Malefic series in order to use it, but when you do, you have a super-strong card in both attack and defense points. The biggest issue with this card is that it can only stay in play if there is a field spell card, which is a much easier target than its 5000 defense points, and it takes up half your life points to summon.
4 Dragon Master Knight (4000 ATK, 4000 DEF)
Dragon Master Knight needs a long time to set up with Blue-Eyes Ultimate Dragon and Black Luster Soldier to be in the same place. It's arguable that it would be overkill, but that's exactly what you're looking for. To add to the madness of this card, it gains 500 attack points for every dragon card on the field. If your opponent makes the mistake of flipping it back into attack mode they would get a devastating consequence.
3 Five-Headed Dragon (5000 ATK, 5000 DEF)
Five-Headed Dragon is going to be one of the best defense cards you can get, believe it or not. You might think it was only good for attacking with its super high strength, but it has that equivalent defense, and to add onto this it can't be killed by five different types. This thing is nearly unkillable, even if it can be killed it will be a resource-draining fight for your opponent to do so.
2 Dystopia The Despondent (5000 ATK, 5000 DEF)
Quite simply, this card cannot be destroyed in battle. It isn't awful to summon either if you have a multi-Kuriboh type deck to use as sacrifice tokens for this card, as it requires 4 level-one monsters to be sent to the graveyard. No other monster on your side can attack so it makes this card much more attack based but your opponent will have a hell of a time trying to get rid of this thing.
1 Don Thousand Monster C (100,000 ATK, 100,000 DEF)
This special card is for a now-defunct style of Yu-Gi-Oh! battle known as boss battles, which was supposed to feature several players fighting one card. This card is nearly impossible to beat and equally as hard to find. With 100,000 Defense points, it is literally impossible for even the best player to beat in battle. The use of this card, however, is banned in official Yu-Gi-Oh! battles, meaning you can only have fun with it with battles against a friend. This is officially the strongest Yu-Gi-Oh! card in both attack and defense in existence.